Field of the Invention
This invention relates generally to amplifier circuits, and more particularly to a differential amplifier having field effect transistor current source loads in the input stage thereof.
Prior Art
It is known that operational amplifiers with field effect transistor (FET) input stages are significantly faster than amplifiers with bipolar transistor input stages. This greater speed of the FET input stages results from the inherently lower ratio of transconductance-to-operating current in the FET as compared to the bipolar transistor.
In addition, the well known use of bipolar current source loads for the differential pair of an input stage of an amplifier results in high input noise, offset and drift. This occurs because the noise and offset of the bipolar current source load devices when referred to the input of the amplifier are amplified by the ratio of the bipolar transconductance to the input stage transconductance.
The use of resistor loads for an FET input amplifier does not introduce noise or drift, but results in a severe compromise between the range of the input voltage which can be employed and the gain of the amplifier. It can be appreciated that it is desirable to obtain a maximum input voltage range and a maximum amount of gain without one compromising the other.
Accordingly, it can be appreciated that a need exists for an amplifier input stage in which a compromise need not be made between the input voltage range and the gain thereof. Furthermore, a need exists for an amplifier input stage having less input noise, offset and drift than that found in bipolar current source loads.